Datadog: Add router checks
Datadog: Add router checks

1. Healthy router check
   - if at least 50% routers fail healthcheck. This is useful to compare
     with manifest update policy.

2. gorouter process running
   - whether the gorouter process is running. This can die without
     datadog agent dying as well. Alert separately.

3. total routes difference
   - alert if we get dramatic drop (more than 33%) in number of routes.
     This can be because of some component failing to update routes. It is
     unlikely that we'll shut down more than 33% of apps at the same
     time.

4. route update latency
   - alert if the router has missed 2 update cycles (every 20s). This
     can indicate some network problems or issues with components that update
     routes.

5. total routes discrepancy
   - outliers detection for routers that have significantly different
     number of routes than average

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf